OBJECTIVE: To assess the feasibility and outcome of measuring the theory of planned behaviour (TPB) in patients receiving routine counselling versus counselling with a decision aid (DA) during primary care consultation on cardiovascular risk prevention. METHODS: A DA was developed, based on models of shared decision-making (SDM) and the TPB. We evaluated the impact of the intervention in a randomized controlled trial. Main outcomes were previously reported. To assess the intermediate social cognitive processes and our theoretical framework, we evaluated the impact of the intervention on a TPB scale. RESULTS: The TPB scale showed satisfactory measurement properties. Factor analysis (main component analysis, confirmatory model) could mostly replicate the assumptions of the model. 44% of variance of the behavioural intention to adhere to the decision after counselling was explained in linear regression models. Of the TPB components, only attitude towards the decision and moral norm were significantly more positive in the intervention. No difference was found with regard to intention to adhere to the decision. High risk resulted in higher values of the TPB components in both groups. CONCLUSION: Most DAs are developed and tested without explicitly referring to a theoretical model of psychosocial processes. The TPB may serve as a useful theoretical framework. PRACTICE IMPLICATIONS: Trials on DAs demonstrate positive effects on psychological outcomes of patients without leading to better objective health results. Our study might contribute to an explanation: DAs might not cause stronger adherence to decisions even though one's attitude towards the decision becomes more positive.